This invention relates to the drilling of holes, using laser radiation into components having an internal cavity.
Laser radiation is often used for the precision drilling of small holes in components used, for example, in the aerospace industry.
Two methods for producing such holes with laser radiation are available; direct drilling and trepanning. Direct drilling involves the use of a "pulsed" laser wherein material is removed little by little by individual pulses of laser radiation concentrated on a small area. The general technique involves directing a number of pulses of laser radiation to burn through the material plus one or more subsequent pulses to "clean out" the interior of the hole and to remove the rough edges left on the hole as the laser emerges through the material.
Trepanning involves direct drilling of a small hole through the material which is then enlarged by moving the laser source and the component relative to one another such that the edges of the hole are gradually removed. During this enlargement process a significant proportion of the energy derived from the laser passes straight through the hole made and is not incident upon the material it is desired to remove.
When drilling holes, by either of the above methods, into components having an internal cavity, a major problem is the prevention of damage to the internal back wall of the cavity upon break through by the laser beam. Such damage is especially prevalent when drilling by the trepanning method during which, only around 50% of the laser energy is incident on the material being removed, the majority of the rest of the energy passing down the already drilled hole which acts as a "light pipe", maintaining a constant beam width and so tending to maximise the backwall damage.
It is known to use paraffin wax containing up to 60% by weight PTFE as a "laser-stopper" material to fill the cavity of such components prior to laser drilling. Such PTFE-filled wax disperses the laser energy incident upon it and thereby prevents or reduces damage to the cavity backwall. However, problems associated with the use of PTFE-filled wax are that the wax tends to melt as the component heats up and may drip out of the holes already drilled; since laser drilling is often carried out in an oxygen-rich atmosphere the wax is prone to burning as it drips out of the component. Furthermore such waxes also tend to leave a carbon-rich deposit or "char" on the component which may chemically combine with the remelted metal at the hole edges and can lead to cracking and general weakening of the component.
It is an object of the present invention to provide an improved cavity filling material, for use in the laser drilling of holes in components having internal cavities, to minimise damage occurring to the backwall of the cavity when the laser beam breaks through the component surface.
It is a further object to minimise the other undesirable chemical side effects which occur with known filling materials.